Torque sensors configured to detect a torque acting around a predetermined rotation axis have been widely used for a variety of transport machinery and industrial machinery. For example, the following Patent Literature 1 discloses a torque sensor of a type in which mechanical deformation caused by action of a torque is detected by a strain gauge. In addition, Patent Literature 2 discloses a sensor which detects a torque acting on a shaft by forming a magnetostrictive film through plating on a shaft surface and measuring a change in magnetic properties of the magnetostrictive film. Meanwhile, Patent Literature 3 discloses a torque sensor of a type in which a magnetic force generating part is provided at an end portion of a torsion bar, and a change in magnetic flux density of a magnetic force generated by the magnetic force generating part is detected using a magnetic collecting ring, and Patent Literature 4 discloses a torque sensor of a type in which a large number of magnets are arranged in a cylindrical shape such that N poles and S poles are alternately arranged in the circumferential direction and a magnetic field generated by these magnets is detected. Further, Patent Literature 5 discloses a torque sensor in which a link mechanism that deforms a shape of an annular member in a radial direction by action of a torque is prepared and a force applied in the radial direction caused by deformation of the annular member is detected by a load sensor. In addition, Cited Document 6 discloses a torque sensor of a capacitance type that detects a torque based on a variation amount of a capacitance value of a capacitive element caused by deformation generated in an annular elastic ring by action of a torque.
In recent years, the application of these torque sensors to life supporting robots has spread, and there is a demand for high safety. However, a current capacitance type torque sensor is provided with a mechanism including a torque detection portion, a CV conversion circuit, and an electronic circuit including a microcomputer, for example, and is likely to malfunction due to condensation, an impact, an overload, or a contamination that is mixed between a pair of parallel flat plates to provide the capacitance. In particular, a torque detection portion of the torque sensor has flexibility, and thus, metal fatigue is caused by the overload or a repeated load. As a result, a crack or the like may be generated in an elastic body forming the torque detection portion, and there is a risk that the elastic body may be eventually broken.
As a simple method of determining whether a torque sensor malfunctions, disclosed is a method of arranging a plurality of (for example, three) torque sensors described in Cited Document 6 in parallel along a rotation axis of a torque, which is an object to be detected, and evaluating a difference between output signals of the respective torque sensors. In this method, three output signals are compared two by two, and when a difference between output signals of two torque sensors falls within a predetermined range, it is determined that the torque sensors normally function. On the other hand, when the difference does not fall within the predetermined range, it is determined that the torque sensors do not normally function (malfunction).